2.1 Classification of reproducing physical quantities units and systems for transferring their sizes (RUTS)and systems for transferring their sizes (RUTS)
2.1.5 Some problems of the specific classification of particular RUTS
2.1.5.3 The problem of “physical constants” in the RUTS systems
Physical constantsare also physical quantities and also have to be measured. Natu- rally, the question arises: What is their relation to the RUTS systems? Or, to be more precise: What is the type of the RUTS system they can belong to?
An analysis of operating national verification schemes shows that there are already about 15 RUTS systems and their subdivisions for the quantities of such a kind which are, as a matter of fact, the physical constants (wavelengths for optical radiation, den- sity and viscosity of liquids, coefficient of linear heat expansion, specific heat and heat conductivity of solid bodies, heat of various phase transfer of substances, rotation an-
Section 2.1 Classification of RUTS systems 89 gle of the polarization plane of optical radiation in a substance, dielectric permittivity, etc.).
However, these systems, constructed in the form of systems of the CC type (with a national measurement standard at the head), were created under the influence and by analogy with the RUTS systems which had been already created for “nonconstant”
physical quantities without any proper reasoning.
First of all, let us note that the hierarchical property of physical quantities, which was discussed above, is especially obvious when one deals with physical constants.
There are constants of afundamentalcharacter (the so-calleduniversal constantsor fundamental physical constantsor fundamental physical constants(FPC)) and less universal(local) constants, i.e.,constants of concrete substances(objects).
Among other things, it is impossible to draw a distinction between them. So, in many investigations and calculations the acceleration of gravity for the earth is assumed to be a universal constant, while in other cases (requiring a greater accuracy) it is assumed to be a constant of especially local significance (at a given point on the earth and under a constant environment).
Another important moment is thepermanency of constants(even the universal ones) which in its absolute sense is onlyhypothetical. Experiment (absolute measurements of their values) gives the actual information about how much they are permanent (in time, space as well as under the influence of some fields). At the same time at present a number of constants (more or less universal) are used for reproducing units ofother physical quantities, their values being considered asfixed.
This is caused, first of all, by the fact that within the space and time limits of the measurement system, where these constants are used for reproducing the units, their permanency has been really determined with high accuracy.
However, the (a priori) fixation of a dimensional constant value means the intro- duction of a new unit into the system of units [432, p. 31]. Therefore, if the constant Ckis used for reproducing a unit of some other physical quantity'iaccording to the equation of indirect measurements (2.11), then this PQ thereby becomes one of the basic ones. Thus, for example, the value of the constant 2heand that of the constant0
are fixed when reproducing the EMF (Volt) and electrical current units, respectively.
The physical constants, however, are frequently used when reproducing physical quantity of the same nature, i.e., the nature uniform with respect to the constant it- self. In exactly the same way the physical constants are used in national measurement standards of frequency and time units (where the frequency of transition between cer- tain levels of a137Cs atom is fixed; generally speaking this frequency is an especially
“local” constant having a high stability), length (where the wavelength of a certain monochromatic electromagnetic radiation is fixed), temperature (where the tempera- ture of one of the phase transitions of a certain substance is fixed), density and viscosity (where the values of corresponding quantities for water under certain conditions are fixed), and others.
It is of the essence that the fixation in these cases can differ. If the size of a constant is fixeda priori andformally(only through the definition of the unit), then in this case the corresponding unit becomes one of the basic units (the cases with frequency, length, temperature).
It is clear that the physical constants of such a type (leading to the basic units) can have only RUTS systems of the CC type and coinciding with the RUTS system corresponding to thebasicphysical quantity (uniform in respect to the constant).
For example, the RUTS systems for length (meter) and wavelength in spectroscopy have to coincide. If the constantvalueis fixed (i.e., a priorimeasurementinformation about its unit is used), then in this case according to the above the constant given can, strictly speaking, not be used for reproducing the unit of the given physical quantity.
This will not be theunit reproduction.
However the reproduction of the density and viscosity units with the help of corre- sponding national measurement standards does not contradict the above, since these national standards function within the frames of one system of measurements, and the
“standardized” values of water density and viscosity are obtained from measurements carried out within the framework of the other system of measurements, i.e., the inter- national one (although in these cases the choice of the RUTS system of only the CC type is not well-founded).
This is the very essence of the work connected with the State Service of Standard Reference Data (SSSRD) which is an additional subsystem with respect to the RUTS system within the framework of the system of enshuring uniformity measurement in the country (Figure 2.2).
At the same time if the Ck value is fixed within the frames of the national sys- tem of measurements, then the improvement of theCk value, which can be realized by indirect measurements of other physical quantities, can only take place within the framework of the international RUTS system. This is what thereversibilityof a mea- surement standard setup consists of; either the reproduction of a unit given (with a system of the CC or LC) on the basis of an international value of the constant, or the improvement of the constant on the basis of its international reproduction (with a system of the LC type).
From this it follows that for constants and other PQs,any type of RUTS system(CC, MC, LC, DC) is possible. The choice will be determined only under considerations of necessity and possibility (except the case leading to the basic PQ units).
However, if for physical constants leading to the basic units such systems can be only the systems of the CC or MC type, then for the remaining cases (density, viscosity, TCLE, etc.) such a type of the system is most improbable. This is seen from the fact that the physical constants are stable in time, i.e., measurements are not numerous and they are carried out infrequently (the number of working measuring instruments is small). Further, practically no new instruments are created for its direct measurements (with the exception of density and viscosity).
Section 2.1 Classification of RUTS systems 91 Systems of the MC type can arise if the physical constant has to be measured under different conditions (for example, the TCLE, specific heat capacity, thermal conduc- tivity of solid bodies at different temperatures).
Finally, the most important thing is that the ability of the “standardized” interna- tional value of the physical constant allows this value to be used for reproducing the unit in any setup suitable for measuring the PQ by an indirect method (i.e., the real- ization of a RUTS system of the LC or even DC type is economically simple and easy to do).